Recently, there is increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development for them. In this regard, electrochemical devices are the subject of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention.
Among the currently used secondary batteries, lithium secondary batteries, developed in early 1990's, have a drive voltage and an energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries, Ni—Cd batteries and H2SO4—Pb batteries), and thus are spotlighted in the field of secondary batteries. However, lithium secondary batteries have problems related to their safety, due to ignition and explosion caused by the use of organic electrolytes, and are manufactured by a complicated process. Lithium ion polymer batteries, appearing more recently, solve the above-mentioned disadvantages of secondary lithium ion batteries, and thus become one of the most potent candidates of next generation batteries. However, such secondary lithium ion polymer batteries still have low capacity compared to secondary lithium ion batteries. Particularly, they show insufficient discharge capacity at low temperature. Hence, there is an imminent need for the improvement of secondary lithium ion batteries.
A lithium ion battery is manufactured by coating a cathode active material (e.g. LiCoO2) and an anode active material (e.g. graphite), which have crystal structures including interstitial volumes, onto the corresponding current collector (i.e. aluminum foil and copper foil, respectively) to provide a cathode and an anode. Then, a separator is interposed between both electrodes to form an electrode assembly, and an electrolyte is injected into the electrode assembly. During a charge cycle of the battery, lithium intercalated into the crystal structure of the cathode active material is deintercalated, and then intercalated into the crystal structure of the anode active material. On the other hand, during a discharge cycle, lithium intercalated into the anode active material is deintercalated again, and then intercalated back into the crystal structure of the cathode. As charge/discharge cycles are repeated, lithium ions reciprocate between the cathode and the anode. In this regard, a lithium ion battery is also referred to as a rocking chair battery.
Such batteries have been produced by many battery producers. However, most lithium secondary batteries have different safety characteristics depending on several factors. Evaluation of and security in safety of batteries are very important matters to be considered. Particularly, users should be protected from being damaged by malfunctioning batteries. Therefore, safety of batteries is strictly restricted in terms of ignition and combustion of batteries by safety standards.
Many attempts have been made to solve the problem related to the safety of a battery. However, ignition of a battery, caused by a forced internal short circuit due to external impacts (particularly, in the case of a customer-abused battery) cannot be solved yet.
Recently, U.S. Pat. No. 6,432,586 discloses a polyolefin-based separator coated with an inorganic layer such as calcium carbonate, silica, etc., so as to prevent an internal short circuit, caused by dendrite growth inside of a battery. However, in case of adopting such an inorganic composite layer, the battery, compared with the conventional battery using a polymer separator, gets heavier and its quality is deteriorated. In particular, since a part of non-porous inorganic particles in the inorganic material layer influences as resistance to the movement of lithium ions that determines quality of a battery, it is fundamentally not possible to avoid the quality deterioration of the battery. Moreover, an increase in weight by the inorganic material layer causes a decrease in energy density of the battery per unit weight. If the inorganic substance content in the coating layer is reduced to solve this, however, it poses another problem that a satisfactory level of an internal short circuit prevention function is not obtained.
Meanwhile, the international union of pure and applied chemistry (IUPAC) defines a pore of 2 nm or shorter in diameter as a micropore, a pore of 2 to 50 nm in diameter as a mesopore, and a pore of 50 nm or greater in diameter as a macropore. Porous materials are expected to hold interest continuously not only for industrial applications but also for academic aspects. Pores are something to be removed in the field of powder metallurgy to obtain a sintered compact, and regarded as defects to be controlled in a casting process to manufacture a sound casting. Nevertheless, a porous material having pores of uniform size and regular arrangement is continuously utilized in various industries that appreciate adsorption and separation efficiency of the porous material. Manufacturing methods of such porous materials include a self-assembly technique, an aerogel manufacturing technique through a sol-gel process, an anodic oxidation technique of aluminum, a condensation drying technique and the like. However, these techniques are mainly used for manufacturing films or monolith porous materials, not for particles.